XULRunner is a single "gecko runtime" that can be used to bootstrap
multiple XUL + XPCOM applications that are as rich as Firefox and
Thunderbird.
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XUL is Mozilla's XML based User Interface language that lets you build
feature-rich cross platform applications. These applications are easily
customized with alternative text, graphics and layout so that they can be
readily branded or localized for various markets. Web developers already
familiar with Dynamic HTML (DHTML) will learn XUL quickly and can start
building applications right away.
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XPCOM is a cross platform component model. It has multiple language
bindings and IDL descriptions so programmers can plug their custom
functionality into the framework and connect it with other components.